FIG. 1 shows a ducted fan gas turbine engine 10 comprising in axial flow series: an air intake 12, a propulsive fan 14 having a plurality of fan blades 16, an intermediate pressure compressor 18, a high-pressure compressor 20, a combustor 22, a high-pressure turbine 24, an intermediate pressure turbine 26, a low-pressure turbine 28 and a core exhaust nozzle 30. A nacelle (not shown) generally surrounds the fan casing 32 and engine 10 and defines the intake 12, a bypass duct 34 and a bypass exhaust nozzle. The engine has a principal axis of rotation 31.
Air entering the intake 12 is accelerated by the fan 14 to produce a bypass flow and a core flow. The bypass flow travels down the bypass duct 34 and exits the bypass exhaust nozzle 36 to provide the majority of the propulsive thrust produced by the engine 10. The core flow enters in axial flow series the intermediate pressure compressor 18, high pressure compressor 20 and the combustor 22, where fuel is added to the compressed air and the mixture burnt. The hot combustion products expand through and drive the high, intermediate and low-pressure turbines 24, 26, 28 before being exhausted through the nozzle 30 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 24, 26, 28 respectively drive the high and intermediate pressure compressors 20, 18 and the fan 14 by concentric interconnecting shafts 38, 40, 42.
The functional requirements of the fan structure and transmission systems of the fan include amongst others: reacting the fan thrust, radial and couple loads; transmitting the power from the turbine to the fan; and transferring structural loads to the engine casing, nacelle and ultimately airframe.
The loads from the fan rotor are transmitted to the engine structure by the use of bearings. The bearings and general shafting arrangement are a key component to address the reaction of loads and transmitting of power to the fan from the turbine.
Typically, the LP system of a direct drive turbofan such as that shown in FIG. 1 consists of fan and turbine rotors connected by a shaft which is supported in the engine structure by a combined bearing support system. The bearing support system usually comprises two or three bearings for the whole LP system. The bearings are typically positioned towards the ends of the respective shaft and optionally at a mid-portion depending on the specific requirements of the engine.
Current trends in gas turbine engines are moving towards so-called geared turbofan engines in which the fan is driven through a reduction gear train. The gear train allows the low pressure spool to be driven at higher rotational speeds which provides for a more efficient lighter engine core, whilst reducing the speed of the fan allows it to be a larger diameter thereby providing a higher bypass ratio. The reduction gear trains may be epicyclic in which the fan is driven via the carrier of a planetary configuration or a star configuration in which the planet gears are fixed, the fan shaft being driven by the ring or star gear. The gear train may be a compound configuration as known in the art.
EP1777380 describes a counter rotating fan assembly coupled to the low pressure turbine. The fan assembly includes a first fan assembly which is directly connected to and driven by the low pressure turbine, and a second fan assembly. The second fan assembly is driven through a gearbox to provide the counter rotation. The second fan is also driveably connected to the low pressure compressor or so-called booster compressor.
The introduction of the reduction gearing leads to a more complex bearing support system in which the low pressure spool, gear train and fan all require bearing support.
The present invention seeks to provide an improved shafting arrangement which allows for improved bearing support.